Silicone is a material of polymeric nature whose chains are made up of alternating oxygen and silicon atoms. Silicones are normally prepared by hydrolysis and subsequent polymerisation of alkylhalogensilanes (both acid- and base-catalysed). The alkylhalogensilanes are in practice made by a direct process, Cu-catalysed, in which the Si reacts with the corresponding alkyl halide. This process provides mixtures of products, whose composition can be modified by a process of redistribution to yield the desired monomer.
Known in the art are silicone elastomers, which are made up of linear polymers. A cross-linking phase is required in order to provide the elastic properties. The most common elastomers are those deriving from dichloromethylsilane, with molecular weights ranging between 300,000 and 700,000. They are made by a prepolymerisation that provides octamethylcyclotetrasiloxane, purification thereof and subsequent polymerisation in the presence of a small quantity of monofunctional material in order to control the molecular weight, followed by a cross-linking similar to curing, in the presence of peroxides, which lends the material its elastic properties. Other important elastomers are those that contain a small proportion (0.1% molar) of vinyl groups linked to silicon, which undergo much more effective curing, and those that contain between 10 and 15% molar of phenyl groups, and good elastic properties at low temperatures. Elastomers of a much lower molecular weight (10,000 to 100,000) can be obtained by using linear polymer chains ending in silanol groups, which can be cured at room temperature by reaction with an alkoxylane.
In general, the most important characteristic of the silicone elastomers is the fact that they present a very broad thermal spectrum of use (from -50.degree. C. to 200.degree. C.) without a significant alteration of their properties. They have good electrical insulation properties, do not self-oxidise or undergo attack by chemical agents in aqueous medium and swell in the presence of non-polar organic solvents, although some special types that contain fluoro- or cyano- groups offer greater resistance to this process.
Silicone elastomers find their widest industrial application as electrical insulators, fluid-repellents and oxidation protectors, and in the manufacturing of hermetic gaskets. The silicones are highly inert materials, and they repel water. Silicone is inert to chemical agents, with the exception of strong bases and acids, and its toxicity is generally low. The origin of these properties lies essentially in the high stability of the Si--O bond (106 Kcal/mol), and in its strong partial ionic character.
Other known uses of silicones are in the manufacturing of containers for liquids (such as wineskins) and tubes for transporting substances (such as the tubes used for blood transfusions).